Specific mail on newsletter signup

[Pete Nice]

So here's what I'm trying to achieve:

Supply a voucher code if you signup to the newsletter when you set up your account.
I can add the code to non-registered users email but how can I send a specific email if you check the newsletter box on sign up?

Any help would be appreciated.

[Pete Nice]

For anyone that may want to solution - here's what I did:

In authentication.php I've added in the first line to the script where it checks if the new user has checked the newsletter box - $customer->newsletter_check = "Your 15% Off Voucher code is: NL01";:

$customer = new Customer();
       if (Tools::isSubmit('newsletter'))
       {

          $customer->newsletter_check = "Your 15% Off Voucher code is: NL01";
           $customer->ip_registration_newsletter = pSQL(Tools::getRemoteAddr());
           $customer->newsletter_date_add = pSQL(date('Y-m-d H:i:s'));
       }

Then added that element in to the array to output to the email that gets sent - {newsletter}' => $customer->newsletter_check :

if (!Mail::Send(intval($cookie->id_lang), 'account', 'Welcome!', 
                       array('{firstname}' => $customer->firstname, '{lastname}' => $customer->lastname,'{newsletter}' => $customer->newsletter_check, '{email}' => $customer->email, '{passwd}' => Tools::getValue('passwd')), $customer->email, $customer->firstname.' '.$customer->lastname))

Then in my email template added {newsletter} where I wanted it to echo out.